In order to provide ease of masonry construction work at site and to reduce the period of time required for the construction work, a masonry wall structure composed of hollow concrete blocks is in wide use typically as a retaining wall which is a wall for sustaining the pressure of earth behind the wall structure. Among such hollow concrete blocks is known a built-up type reinforced-concrete block which is composed of initially separate component panels. The reinforced-concrete component panels for such a built-up masonry block unit are prefabricated as separate members and are combined together into hollow block form at the site of construction. Each of the component panels of the block units is thus provided with strips or bands of steel which are to be bolted to the steel strips or to bands of another panel. Large amounts of time and labor are required for the assembling of the block at the site of construction in addition to the considerable amount of costs for the purchase, or manufacture, of the steel bands, bolts and nuts per se which are used additionally to the ordinary reinforcing bars embedded in the panels. This significantly impairs the potential advantages of the hollow built-up concrete blocks which are easy to manufacture, transport and handle.
The concrete panels to be combined together by bolting the steel bands have another problem, which results from the fact that such panels must be formed with cutouts or recesses to provide spaces reserved for accommodating bolts and nuts where interlocking joints are to be formed between the panels. Provision of such cutouts or recesses significantly restricts the geometry and design considerations of the concrete panels so that panels with two different designs must be provided and used, the panels with one design being for one side of a block unit and the panels with the other design being for the other side of a block unit. This is objectionable from the view point of simplicity of construction of the block units and of increasing the working efficiency at the site of construction.